The present invention relates generally to telescopic imaging, and more particularly to a new coronagraph for increasing the contrast ratio between the infrared image of a star and that of a possible orbiting planet.
The search by direct telescopic imaging for planets orbiting stars not our sun is extremely difficult because of the angular proximity of such planets to the star around which they orbit and because of the faintness of such planets relative to their stars. The image of a nonsolar planet would typically be separated by only tenths of arcseconds from that of its star and there would be 10.sup.9 more starlight than planet light at visible wavelengths.
The search for so-called nonsolar or extra-solar planets can be made somewhat easier by working at infrared wavelengths, where the contrast ratio between star and planet would be only 10.sup.7.
The contrast ratio can be further increased by selectively suppressing the image of the star using a coronagraph. A coronagraph is most often used to suppress the central disk image of the sun so that its surrounding corona can be seen or imaged. A properly shaped coronagraph "apodizes" the energy in the star's diffraction pattern. Apodization is the process of applying a preselected function to a waveform to gradually reduce its amplitude as it approaches a certain point. Apodization can be performed mathematically, optically, or mechanically. A coronagraph generally optically and/or mechanically apodizes the sun's image in a manner which can be described mathematically.
The best known example of a coronagraph is the standard Lyot coronagraph, first described by M. B. Lyot. The Lyot coronagraph, shown in FIG. 1 of the drawings and described in more detail in the Detailed Description of the invention, combines a centrally positioned first apodizer, for blocking the direct light from a star, with an annular mask, called a Lyot stop, for blocking the star's diffracted light.
Unfortunately, even prior art improved versions of the Lyot coronagraph (using a Gaussian first apodizer) cannot increase contrast ratios sufficiently to discern, or image, a planet having a relative irradiance of 10.sup.-7.
Thus it is seen that there is a need for a new coronagraph with improved performance sufficient to make possible detection of nonsolar planets.
It is, therefore, a principal object of the present invention to provide new shapes for coronagraph apertures, apodizers and Lyot stops that can suppress stellar irradiance sufficiently to directly image a planet in proximity to its parent star.
It is a feature of the present invention that it can use the relatively bright on-axis point source which is otherwise deliberately discarded by the coronagraph as a reference signal to maintain alignment of a multi-segment telescope mirror.
It is an advantage of the present invention that its new shapes for coronagraph apertures, apodizers and Lyot stops also allow greater amounts of planet irradiance to propagate through the imaging system.
It is another advantage of the present invention that it is simple to understand and will be simple and straightforward to make and to use.
These and other objects, features and advantages of the present invention will become apparent as the description of certain representative embodiments proceeds.